Multi-band radio receivers, that is to say radio receivers that are capable of receiving signals in two or more frequency bands, may be implemented using integrated circuits (chips) in devices such as mobile telephones and the like.
Such receivers often require filters external to the chip to reduce the effects of strong out-of-band blocking signals on the performance of the chip. Such blocking signals may arise, for example, from transmitters located close to the chip in a device in which the chip is used.
Typically, SAW (surface acoustic wave) filters are used to filter out such out-of-band blocking signals. A typical receiver architecture having three receive paths, only one of which is operational at any one time, is illustrated schematically in FIG. 1. In the architecture 10 of FIG. 1, a multi-resonant antenna 12 is able to receive signals in three different frequency bands. The output of the antenna 12 is connected to parallel SAW filters 14, 16, 18, which are external to a chip in which parallel low noise amplifiers (LNAs) 20, 22, 24 are implemented.
Each of the LNAs 20, 22, 24 is configured to operate in one of the frequency bands of the antenna 12, and receives at its input the output of a respective one of the SAW filters 14, 16, 18, which are configured to pass signals in the frequency band of interest and to attenuate out-of-band signals strongly.
The outputs of the LNAs 20, 22, 24 are fed to an input of a common buffer 26, via a common output resonator or tank circuit made up of a variable capacitor 28 and an inductor 30 connected in parallel between a supply voltage and the input of the buffer 26. The common output tank circuit can be tuned using the variable capacitor 28 to select its centre frequency.
A problem with architectures such as the one illustrated in FIG. 1 is that the bill of materials (BOM) cost can be high, as it is dominated by components external to the chip, and in particular by the SAW filters 14, 16, 18. Thus, it would be desirable to provide a receiver architecture which provides attenuation of out-of-band blocking signals without using costly SAW filters.